The present invention relates to spectrographs, particularly to a spectrograph capable of providing high resolution spectra of the mid-infrared region, and more particularly to a spectrography incorporating a silicon immersion echelle grating and a transmission grating in a cross-dispersing configuration in combination with an infrared detector array.
In recent years substantial effort has been directed to the problem of detection of airborne chemicals. The remote detection of airborne chemicals issuing from exhaust stacks, vehicle exhaust, and various exhaust flumes or plumes, offers a non-intrusive means for detecting, monitoring, and attributing pollution source terms. To detect, identify, and quantify a chemical effluent, it is highly desirable to operate at the limiting spectral resolution set by atmospheric pressure broadening at approximately 0.1 cm.sup.-1. This provides for maximum sensitivity to simple molecules with the narrowest spectral features, allows for corrections for the presence of atmospheric constituents, maximizing species selectivity, and provides greater opportunity to detect unanticipated species. Fourier transform spectrometers, such as Michelson interferometers, have long been the instrument of choice for high resolution spectroscopy in the infrared spectral region. This derives from its advantage in light gathering power and spectral multiplexing over conventional dispersive spectrometers. For remote sensing applications and for those applications in hostile environments, the Fourier transform spectrometer, such as the Michelson interferometer, is ill suited for these applications due to the requirements for keeping a moving mirror aligned to better than a wavelength over the mirror surface. Furthermore, this spectrometer collects amplitude variations over time that are then transformed into frequency information for spectral generation. Consequently, this approach requires stable radiation sources and has difficulty dealing with rapidly changing reflectors or emissions as generally encountered in remote field observations, particularly from moving observation platforms. Furthermore, under conditions where the noise terms are dominated by the light source itself, the sensitivity of the instrument is limited by the so-called multiplex disadvantage.
Dispersive infrared instruments on the other hand acquire spectral information serially in time, are generally much larger instruments at the same spectral resolution, and have less light gathering power. Thus, there has been a need for an instrument that provides simultaneous spectral data collection over a wide spectral band without suffering the multiplex disadvantage.
Recent advances in two-dimensional infrared detector array capacity and performance along with advances in chemical micromachining technology provide the opportunity for creating a new class of remote sensing infrared spectrometers.
For the past several years the Lawrence Livermore National Laboratory (LLNL) has been involved in the development of effluent sensing technology. These development efforts have resulted in a new generation of rugged, high-performance infrared (IR) spectrometers that combine excellent spectral resolution with high reliability and small package size. The LLNL design of these field instruments is based on the precision fabrication of silicon and germanium immersion gratings. It is the high index of refraction of these materials that leads to highly dispersive gratings and enables immersion grating spectrometers to be very compact. Spectrometers capable of mid-IR and longwave-IR coverage can be realized with this new technology. This technology and early design developments over the past several years have led to the design of a compact, rugged cryogenic spectrometer covering the atmospheric transmission bands (K and L bands) between 2.3 and 4.2 microns. The spectrometers are designed for airborne operation. Such early design developments at LLNL are described in an article entitled "Cross dispersion infrared spectrometry (CDIRS) for remote chemical sensing" by C. G. Stevens et al. In addition, document UCRL-JC-120743, "Design of a mid-IR immersion echelle grating spectrograph for remote sensing", N. L. Thomas et al., bearing a date of May 9, 1995, describes the design of a second generation of remote sensing echelle grating spectrometers developed at LLNL and is based on the availability of silicon immersion gratings of high precision. The dispersion of an immersion grating is increased in proportion to the refractive index. For silicon this factor is 3.4, permitting a very significant reduction in the overall size of the spectrometer while maintaining the same resolution and light throughout. The objective of the above-referenced second generation design was to develop a spectrometer covering the mid-infrared atmospheric windows with no moving components by using a cross dispersion approach. The earlier LLNL designs, see above-referenced article by C. G. Stevens et al., utilized an echelle grating crossed with a constant dispersion prism doublet. This provided for very efficient detector array utilization. However, with a smaller immersion grating spectrometer, the need to increase prism dispersion forced a number of design compromises in overall size and system performance. The second generation design, see above-referenced UCRL-JC-120743 is based on the use of a high order silicon immersion grating crossed with a concave grating operating in a Wadsworth configuration.
The present invention satisfies the above-referenced need for an instrument that provides simultaneous spectral data collection over a wide spectral band without suffering the multiplex disadvantage, and like the prior LLNL spectrometer designed, the present invention utilized a silicon immersion echelle grating operating in high spectral order, but in the present invention the silicon immersion grating is combined with a first order transmission grating in a cross-dispersing configuration to provide a two- dimensional spectral format that is focused onto a two-dimensional infrared detector array. In addition, the spectrometer of this invention incorporates a common collimating and condensing lens assembly in a near aberration-free axially symmetric design, which enable a further reduction in size which results in a very small (tiny) immersion echelle spectrograph with no moving components.